1. Field of the Invention
The present invention relates to a thick-film thermal printing head, and more particularly to heat insulating layers and protective layers thereof.
2. Description of the Prior Art
A basic structure of a thick-film thermal head for a thermal printer used in thermal printing comprises, as illustrated in an example shown in FIG. 1, a ceramic substrate 1, a pair of opposing electrodes 2 formed thereon, a resistor 3 serving as a heat generating element, formed between the electrodes 2 and a protective layer or abrasion-resistant layer 4 formed on the electrodes 2 and the resistor 3 to cover the same. In this type of thermal head, a pulse current is supplied to the resistor 3 to cause it to generate heat so that Joule heat thereof causes to change color of a heat-sensitive paper 6 which is urged to the protective layer 4 by a paper feed roller 5.
A first problem encountered in this type of thermal head is that since heat conductivity of the ceramic substrate (which is usually an alumina substrate or a folsterite substrate) is about 30 times as high as heat conductivity of the protective layer (glass layer) 4 formed on the resistor 3 (e.g. 5.times.10.sup.-2 cal/cm.sec..degree.C. for alumina), most parts of heat generated by the resistor are absorbed by the substrate and only 5-10% of the total heat generated is conducted to the heat-sensitive paper.
Thus, in order to attain a desired optical density for printing, power supplied to the resistor must be increased. However, the increased power leads to the reduction of durability of the resistor (which is usually a ruthenium oxide thick-film resistor), that is, the change of resistance during printing and also leads to the increase of cost due to a larger power supply required. In order to reduce the necessary power for printing, it is the most effective way to minimize the escape of the heat generated by the resistor 3 to the ceramic substrate 1. To this end, in a thin-film thermal head, it has been proposed to form a heat insulating layer 7 of poor heat conductivity between the resistor 3 and the ceramic substrate 1, as shown in FIG. 2. The heat insulating layer 7 is usually made of nonalkali glass having a thickness of approximately 30 to 60 .mu.m. See for example, S. Shibata, K. Murasugi and K. Kaminishi, "New type thermal printing head using thin film," IEEE Transactions on Parts, Hybrids, and Packaging, Vol. PHP-12, No. 3, pp. 223-230 Sept. 1976. In the thick-film thermal head, it is also very effective to form the heat insulating layer 7 in improving heat utilization efficiency during printing, but it has been found that since the materials used and the manufacturing processes differ between the thick-film thermal head and the thin-film thermal head, there is involved a serious problem when the heat insulating layer of the thick-film thermal head is made of the nonalkali glass as in the thin-film thermal head. That is, the thick-film thermal head for thermal printing having heat insulating layer 7 may be manufactured by applying glass paste (consisting of lead borosilicate glass frits and .alpha.-terpineol solution of ethyl cellulose) on the ceramic substrate (comprising 96% of alumina) 1 to a desired thickness by printing or the like, firing the paste to form the heat insulating layer 7, printing silver thick-film conductive paste (consisting of gold powder, bismuth containing-lead borosilicate glass frits and .alpha.-terpineol solution of ethyl cellulose) or Pd-Ag thick-film conductive paste (consisting of palladium powder, silver powder, bismuth containing-lead borosilicate glass frits and .alpha.-terpineol solution of ethyl cellulose) and firing the paste to form the electrodes 2, printing ruthenium oxide thick-film resistor paste (consisting of RuO.sub.2 powder, lead borosilicate glass frits and .alpha.-terpineol solution of ethyl cellulose) and firing the paste to form the resistor 3, and printing thick-film glass paste (consisting of lead borosilicate glass frits and .alpha.-terpineol solution of ethyl cellulose) and firing the paste to form the abrasion protective layer 4. However, it has been found that during the manufacturing process the resistance of the resistor is considerably increased during the firing process by the influence of the heat insulating layer.
For example, when the heat insulating layer 7 is formed by printing and firing the glass paste consisting of lead borosilicate glass frits and an .alpha.-terpineol solution of ethyl cellulose and the resistor 3 is formed by printing and firing the resistor paste (RuO.sub.2 thick-film paste described above), the resistance of the resistor 3 increases approximately ten times as high as that when the heat insulating layer is not formed. It has also been found that the variance of the resistance of the resulting resistor is so large that it cannot be used as the heat generator for the thermal printing head.
A second problem encountered in a thermal head built in a hybrid circuit substrate is that when the electrodes 2 are formed on the heat insulating layer 7 having a thickness of 30 to 60 .mu.m as shown in FIG. 3, the electrodes 2 spread as shown in FIGS. 3 and 4 to cause short of the electrodes. As an approach to solve the above problem, it has been proposed to form the heat insulating layer 7 on the entire surface of the ceramic substrate 1 to prevent a step from being formed between the heat insulating layer 7 and the surface of the substrate 1, but this approach involves a problem that a large amount of expensive glass paste is required and that when semiconductor chips or the like are to be mounted on the substrate it becomes more difficult to dissipate heat generated by the semiconductor chips to the substrate.
A third problem resides in the protective layer. In FIG. 2, since the protective layer 4 serves to prevent the wear loss of the heat generating resistor due to the feed of the heat-sensitive paper 6, the protective layer 4 itself must be highly wear-resistant. However, the glass material usually used for the protective layer 4 (lead borosilicate glass having a softening point of 480.degree.-500.degree. C.) is worn by as much as approximately 10 .mu.m through the paper feed of approximately 5 Km. For the abrasion resistance of the glass material to the paper feed, it is necessary that the glass layer remains after the paper feed of 25 Km, and in order to meet the requirement by the conventional glass, the glass layer thickness of approximately 50 .mu.m is required. However, when the thickness of the glass layer is so increased, it becomes harder for the heat generated in the resistor 2 to be conducted to the heat-sensitive paper 6 because the heat conductivity of glass is low, i.e., 2.times.10.sup.-3 cal/.degree.C.cm.sec, and hence, the power to be supplied to the resistor 3 for printing must be increased. The increase of the power leads to the change of resistance of the resistor 3 and reduction of the durability.
The protective layer 4 has been heretofore formed by applying the glass paste (consisting of lead borosilicate glass frits and an .alpha.-terpineol solution of ethyl cellulose) on the resistor 3 (e.g., the RuO.sub.2 thick-film resistor described above) and the electrodes 2 (e.g., the Ag-Pd electrodes described above) to a desired thickness by screen printing or the like, drying the paste and firing the paste at 500.degree. to 600.degree. C. However, when the glass is fired, the resistor is refired and the resistance of the resistor changes. The change of the resistor occurs when the glass firing temperature exceeds 500.degree. C., and the degree of the change increases as the firing temperatures rises. Accordingly, in order to attain a preset magnitude of the resistance, the resistor must be formed taking the change of resistance by refiring into consideration. This is very troublesome and also difficult to obtain the preset magnitude of the resistance. As the resistance of the resistor varies one from the other, the amount of heat generated when a pulse of a given voltage is applied to the resistor 3 differs are one from the other and hence the optical density for printing is not uniform. Accordingly, it is desirable that the material of the protective layer 4 is one which can minimize the change of resistance of the resistor 3 when the material is fired.
Furthermore, as described above, the material of the protective layer heretofore used has a low heat conductivity such as approximately 2.times.10.sup.-3 cal/.degree.C.cm.sec. so that even if the thickness is limited to 15 .mu.m, only about 10% of the heat generated in the resistor can be utilized to change color of the heat-sensitive paper because of a heat insulating effect of the protective layer. Thus, the heat utilization efficiency is poor.